Solder is used to connect, electrically and mechanically, electrical components of downhole tools used in relation to well sites in connection with hydrocarbon exploration and acquisition. For instance, joints may be created by melting the solder between the surfaces to be joined, and then allowing it to solidify, thereby forming the joint. Traditionally tin-lead (HMP) solders have long been used for their high melting point, narrow melting range, fair wetting, reliability, availability and cost advantages. However, the EU Restriction of Hazardous Substances (RoHS) legislation has banned most lead from electronics, which has consequently led to development of lead-free alternatives to tin-lead solder. Many attempts at finding alternatives for high temperature applications focused on tin-silver-copper alloys (also known as Sn—Ag—Cu alloys, or SAC alloys), due to their higher melting temperature.
Lower silver content SAC alloys, such as Sn-1.0Ag-0.5Cu (SAC105), have been found to perform well in high shock and vibration environments (e.g., exhibiting longer joint life), while higher silver content SAC alloys, such as Sn-4.0Ag-0.5Cu (SAC405), have been found to perform well in high temperature applications (e.g., temperatures (T)>125° C.). While all of these solders have melting temperatures in the range of 215° C.-225° C., those with lower silver content were found to be more resistant to failure by shock and vibration, but also less resistant to failure by creep, temperature aging, or temperature cycling compared to those with higher silver content. With this in mind, Sn-3.0Ag-0.5 Cu (SAC305) has been found to exhibit a compromise between SAC105 and SAC405, and has found widespread usage in many applications. However, the long-term reliability of SAC305 is questionable under harsh environments, particularly those combining high temperature thermal fatigue with mechanical shock/vibration.
It is known to provide a contact surface of a component with a finish layer. The finish, also known as plating or coating, serves to protect the contact surface from oxidation, for example on a lead or termination of the component or a solder pad of a printed wiring board, thereby prolonging shelf life as well as facilitating the assembly process by providing a compatible soldering surface onto which solder can be applied. Additionally, many finishes also form or provide a diffusion barrier that minimizes further interaction of the solder with the metal of the contact surface. However, when exposed to high temperatures for an extended period of time, the diffusion barrier capability of this layer is decreased or eliminated, and the solder reacts with the metal of the contact surface. In this respect, it has been found that during exposure to heat after soldering, i.e. when in use, an interfacial intermetallic layer forms between the finish layer and the solder, as a result of this reaction between the solder and the metal of the contact surface, which reduces the bonding strength of the solder joint as this layer continues to grow.